1. Field of the Invention
The present invention relates to a driving device for driving the permanent-magnet synchronous motor with weakening field control using a voltage-type inverter.
2. Description of the Related Art
FIG. 1 shows the conventional driving device for the permanent-magnet synchronous motor.
In FIG. 1, the driving device comprises a direct current power source 1; a voltage-type inverter 2; an electric current detecting unit 3; a permanent-magnet (PM) synchronous motor 4; a magnetic pole position detecting unit 5; a speed detecting unit 6; a control circuit 7 for the voltage-type inverter 2; and a phase voltage detecting unit 8. The voltage-type inverter 2 converts the direct voltage provided by the direct current power source 1 into a phase voltage, and outputs it to the permanent-magnet synchronous motor 4. The electric current provided from the voltage-type inverter 2 to the permanent-magnet synchronous motor 4 is detected by the electric current detecting unit 3. The magnetic pole position detecting unit 5 detects the position of the magnetic pole of the permanent-magnet synchronous motor 4. The speed detecting unit 6 detects the rotation speed of the rotor of the permanent-magnet synchronous motor 4. The phase voltage detecting unit 8 detects the phase voltage provided from the voltage-type inverter 2 to the permanent-magnet synchronous motor 4.
The control circuit 7 generates gate pulse signals P.sub.u *, P.sub.v *, and P.sub.w * for each phase switching element of the voltage-type inverter 2 based on the torque command .tau.* and outputs them. Described below are the configuration and operation of the control circuit 7.
Assume a d-q coordinate system which is a rotating coordinate system rotating in synchronism with the magnetic flux generated by a permanent magnet, that is, the rotor of the permanent-magnet synchronous motor 4. In the d-q coordinate system, the d axis is a coordinate axis in the magnetic flux direction while the q axis is a coordinate axis in the direction vertical to the d axis.
In the control circuit 7, a 3-phase/2-phase converter 701 converts the phase current detection values i.sub.u and i.sub.w (which are phase current values of the permanent-magnet synchronous motor 4 and are detected by the electric current detecting unit 3) into the direct current detection values I.sub.d and I.sub.q (which are elements in the d-q coordinate system using a magnetic pole position signal .theta.).
A q-axis current command operating unit 702 converts the torque command .tau.* for control of the torque of the permanent-magnet synchronous motor 4 into the q-axis current command value I.sub.q * by multiplying the torque command .tau.* by the inverse number K.sub.T.sup.-1 of the torque coefficient. The deviation between the q-axis current command value I.sub.q * and the q-axis current detection value I.sub.q * from the 3-phase/2-phase converter 701 is calculated by an adder 704q.
A d-axis current command operating unit 703 generates the d-axis current command value I.sub.d * based on the phase voltage detection values V.sub.U, V.sub.V, and V.sub.W detected by the phase voltage detecting unit 8 and the rotation speed .omega. of the rotor detected by the speed detecting unit 6. An adder 704d calculates the deviation between the d-axis current command value I.sub.d * and the d-axis current detection value Id received from the 3-phase/2-phase converter 701.
The deviations output from the adders 704q and 704d are input to the proportional plus integral controllers (PI controller) 705 and 706 in the q-axis current control system and the d-axis current control system respectively for controlling the deviation to be set to zero.
The adders 704q and 704d and proportional plus integral controllers 705 and 706 form direct current control systems for controlling the q-axis current detection value I.sub.q and the d-axis current detection value I.sub.d, which are direct current values, to respectively equal the command values I.sub.q * and I.sub.d *.
The outputs from the controllers 705 and 706 are input to a noninteracting compensation system 707.
In the noninteracting compensation system 707, the voltage drops RI.sub.d and RI.sub.q through the armature resistor R of the permanent-magnet synchronous motor 4, the voltage drops .omega.LI.sub.d and .omega.LI.sub.q through the synchronous reactance .omega.L (L indicates a synchronous inductance), and the inverse activation voltage .omega..o slashed. (.o slashed. is a magnetic flux as an inverse activation voltage constant) are calculated as compensation terms based on the d-axis current detection value I.sub.d, q-axis current detection value I.sub.q, and rotation speed .omega.. These compensation terms are added (as indicated by the symbols shown in the figure) to the outputs of the controllers 705 and 706 to generate the phase voltage command values (d-axis current command value and q-axis current command value) V.sub.d * and V.sub.q * of the d-q axis coordinate system.
The voltage command values V.sub.d * and V.sub.q * are input to a coordinate converting unit 708 and converted into the voltage command vector V* which has an amplitude .vertline.V*.vertline. and are represented in the polar coordinate format using an angle .beta. based on the d- and q-axis.
The amplitude .vertline.V*.vertline. and angle .beta. are input to a pulse-width modulation (PWM) operating unit 709, and a PWM operation is performed using the magnetic pole position signal .theta.. Thus, the gate pulse signals P.sub.U *, P.sub.V *, and P.sub.W * are generated for each of the phase switching elements of the inverter 2.
The voltage-type inverter 2 performs the switching operations according to the gate pulse signals P.sub.U *, P.sub.V *, and P.sub.W * to output the voltage matching the voltage command and drive the permanent-magnet synchronous motor 4.
When the permanent-magnet synchronous motor is driven, only the q-axis current normal to the d-axis current may be introduced with the d-axis set to zero (0) so that all magnetic flux of the permanent-magnet can operate as effective magnetic flux.
However, since the magnetic field of the permanent-magnet synchronous motor is constant, the inductive voltage through the motor becomes higher than the maximum voltage of the inverter if the motor is running at a high speed. As a result, the motor functions as a generator. Thus, the operation speed of the motor is limited.
Therefore, the weakening field control is made to weaken the apparent magnetic field for a high-speed operation by decreasing the negative d-axis current flowing through the motor.
According to the prior art shown in FIG. 1, the d-axis current command value I.sub.d * is set to zero (0) when the permanent-magnet synchronous motor 4 is driven without performing the weakening field control.
The negative d-axis current flows in the permanent-magnet synchronous motor 4 in a weakened field area where the inductive voltage of the permanent-magnet synchronous motor 4 is higher than the maximum voltage of the voltage-type inverter 2. However, in the circuit of the prior art, the proportional plus integral controller 706 is used in the d-axis current control system. Therefore, if there is a deviation between the d-axis current command value I.sub.d * and the d-axis current detection value I.sub.d, then a signal proportional to the time integration of the deviation is output, and the output of the controller 706 becomes infinite. If the limiter exists at the output terminal of the controller 706, the output does not become infinite, but the controller 706 becomes saturated, thereby preventing an output from being appropriately issued. As a result, the prior art has to set the d-axis current command value I.sub.d * to a proper negative value.
Thus, in the prior art, the d-axis current command operating unit 703 obtains the d-axis current command value I.sub.d * by performing an operation according to the following equation (1) using the phase voltage detection values V.sub.U, V.sub.V, and V.sub.W from the phase voltage detecting unit 8, and the rotation speed .omega. from the speed detecting unit 6. EQU I.sub.d *=(RI.sub.q +.omega..o slashed.-V.sub.q)/.omega.L (1)
The motor constants R, L, and .o slashed. in the equation above are normally fixed. However, they vary depending on the temperature. Additionally, since L is much smaller than R and .o slashed., the R and .o slashed. vary with the temperature and greatly affect the value of I.sub.d * when the motor is driven at a low speed, that is, with .omega. set to a small value. As a result, the d-axis current command value I.sub.d * is unstable.
As described above about the prior art, the negative d-axis current flows, when the weakening field control is performed, in the current control system provided with the proportional plus integral controller for both d- and q-axes. To prevent the controller from being saturated, the d-axis current command value I.sub.d * should be calculated by, for example, the equation shown above. Since the motor constant varies depending on the temperature, etc., additional operations are required to obtain a correct result by adjusting the temperature, etc.
Furthermore, the V.sub.q in equation (1) is obtained by converting the phase voltage into the d-q coordinates in the rotating coordinate system. Since massive operations are performed to obtain V.sub.q, a high-speed operating unit is required when a microcomputer practically performs the operations. A temperature detecting unit is further required for the temperature correction for the motor constant, and the phase voltage detecting unit 8 is also required to obtain the d-axis current command value I.sub.d *, thus preventing the system from being successfully developed as a small low-cost device.